1. Field of the Invention
The present invention relates to a power transmission apparatus, and more specifically to a power transmission apparatus provided with viscous coupling suitable for use in four-wheel drive vehicles by which an engine power transmission system can be switched from two-wheel drive to four-wheel drive or vice versa.
2. Description of the Prior Art
FIG. 1 is a diagram for assistance in explaining an engine power transmission system for a four-wheel drive (4WD) vehicle. In the drawing, a drive power generated by an engine 1A is speed-changed by a transmission 3A, and then transmitted from a transfer 5A directly to a front wheel side differential gear 7A and to a rear wheel side differential gear 11A via a propeller shaft 9A. The front wheel side differential gear 7A distributes a transmitted power differentially to two (right and left) front wheels 19A and 17A. The rear wheel side differential gear 11A distributes a transmitted power differentially to two (right and left) rear wheels 27A and 25A.
The power transmission apparatus provided with viscous coupling of the present invention is related to the rear wheel side differential gear 11A shown in FIG. 1.
In conventional four-wheel drive vehicles for transmitting torque from an internal combustion engine 1A to front and rear wheels, there exists a problem in that braking phenomenon occurs when the vehicle is driven along a sharp corner as when the vehicle is being put into a garage. To overcome this problem, the four-wheel drive vehicle is usually provided with a differential gear 11A for absorbing a difference in rotational speed between the front and rear wheel drive shafts.
In this four-wheel drive vehicle, when the vehicle travels on a muddy road surface (whose friction coefficient is small) and therefore a front wheel slips, only a small torque (required for a slipping front wheel whose friction coefficient is small) is transmitted from the internal combustion engine 1A to the front and rear wheels in dependence upon the differential gear construction, so that it is impossible to extricate the vehicle out of the muddy road. To overcome this problem, the four-wheel drive vehicle is usually provided with a viscous coupling including a differential limiting device by which differential operation of the differential gear is limited so that the vehicle can be extricated out of a muddy road, as disclosed in "Automobile Engineering" page 62, June, 1987.
On the other hand, a part-time four-wheel drive vehicle is usually provided with a transfer 5A by which two-wheel drive can selectively be switched to four-wheel drive or vice versa. However, when the two-wheel drive is selected through the transfer 5A, the driven side power transmission system from the transfer to the rear wheels is rotated by the running vehicle. Once the driven side power transmission system is rotated, there exist problems in that vehicle travelling resistance increases and therefore vehicle vibration and noise increase. To overcome the above-mentioned problems, there has been proposed a power interruption apparatus for an four-wheel drive vehicle, by which a part of the driven side power transmission system is disconnected, when the vehicle is driven in two-wheel drive mode, to allow the driven side thereof to be free in order that the above-mentioned travelling resistance, vibration and noise can be reduced as low as possible, as disclosed in Japanese Published Unexamined (Kokai) Pat. Application No. 60-215428.
In the former prior-art four-wheel drive vehicle provided with a viscous coupling, although difference in the number of revolutions between the front and rear wheel drive shafts can be absorbed to allow the vehicle from being driven away from the muddy road, since the four wheels are always driven by the engine, there exists a problem in that the fuel consumption rate is high during vehicle travelling.
On the other hand, in the latter prior-art four-wheel drive vehicle provided with a power interruption apparatus, since the vehicle is driven in two-wheel drive mode, it is possible to improve the fuel consumption rate and also reduce the travel resistance, vibration and noise due to the rotation of the driven side power transmission system. However, there exists a problem in that braking phenomenon occurs when the vehicle is driven along a sharp corner and therefore the vehicle cannot travel smoothly because the difference in the number of revolutions between the front and rear drive shafts cannot be absorbed during four-wheel drive travelling.
Further, Japanese Published Unexamined (Kokai) Patent Application No. 60-172764 discloses a power transmission apparatus usable for use in four-wheel drive (4WD) vehicles. This prior-art apparatus is disposed between a suboutput shaft of a transaxle and a rear wheel driving apparatus, and constructed in such a way that an output shaft of the viscous coupling is connected to the rear wheel driving apparatus and the suboutput shaft of the transaxle is connected to or disconnected from input and output shafts of the viscous coupling.
In this prior-art apparatus, three drive modes can be obtained as follows: (1) when the suboutput shaft of the transaxle is disconnected from the input shaft of the viscous coupling, a two-wheel drive (2WD) can be attained; (2) when the suboutput shaft of the transaxle is connected to the input shaft of the viscous coupling, an automatic four-wheel drive (4WD) can be attained (a torque corresponding to a difference in relative revolution speed between the input and output shafts of the viscous coupling can be transmitted); and (3) when the suboutput shaft of the transaxle is connected to the output shaft of the viscous coupling, a direct coupling four-wheel drive (4WD) can be attained.
In this prior-art apparatus as described above, however, there still exist problems in that since the viscous coupling and the rear wheel drive apparatus are rotated by the rear wheels even during the two-wheel drive (2WD) operation, the fuel consumption rate is high and further vibration and noise are produced due to the agitation resistance of the differential gear oil.